year/years

[mollsen]

Why in the English language do we say the following:

I have a two year old and a five year old son.

Two and five are plural, why not use years. Secondly, when asked our age we say years:

I am five years old.

 

[2006]

Why in the English language do we say the following:

I have a two year old and a five year old son.
Two and five are plural, why not use years. Because 'two-year-old' is a compound adjective/noun.
I have a five-year-old (son).

I am five years old. "five years" is how old you are.

2006